Broadcasting and telecommunications have historically occupied separate fields. In the past, broadcasting was largely an “over-the-air” medium while wired media carried telecommunications. That distinction may no longer apply as both broadcasting and telecommunications may be delivered over either wired or wireless media. Present development may adapt broadcasting to mobility services. One limitation has been that broadcasting may often require high bit rate data transmission at rates higher than could be supported by existing mobile communications networks. However, with emerging developments in wireless communications technology, even this obstacle may be overcome.
Terrestrial television and radio broadcast networks have made use of high power transmitters covering broad service areas, which enable one-way distribution of content to user equipment such as televisions and radios. By contrast, wireless telecommunications networks have made use of low power transmitters, which have covered relatively small areas known as “cells”. Unlike broadcast networks, wireless networks may be adapted to provide two-way interactive services between users of user equipment such as telephones and computer equipment.
The introduction of cellular communications systems in the late 1970's and early 1980's represented a significant advance in mobile communications. The networks of this period may be commonly known as first generation, or “1G” systems. These systems were based upon analog, circuit-switching technology, the most prominent of these systems may have been the advanced mobile phone system (AMPS). Second generation, or “2G” systems, ushered improvements in performance over 1G systems and introduced digital technology to mobile communications. Exemplary 2G systems include the global system for mobile communications (GSM), digital AMPS (D-AMPS), and code division multiple access (CDMA). Many of these systems have been designed according to the paradigm of the traditional telephony architecture, often focused on circuit-switched services, voice traffic, and supported data transfer rates up to 14.4 kbits/s. Higher data rates were achieved through the deployment of “2.5G” networks, many of which were adapted to existing 2G network infrastructures. The 2.5G networks began the introduction of packet-switching technology in wireless networks. However, it is the evolution of third generation, or “3G” technology that may introduce fully packet-switched networks, which support high-speed data communications.
Standards for digital television terrestrial broadcasting (DTTB) have evolved around the world with different systems being adopted in different regions. The three leading DTTB systems are, the advanced standards technical committee (ATSC) system, the digital video broadcast terrestrial (DVB-T) system, and the integrated service digital broadcasting terrestrial (ISDB-T) system. The ATSC system has largely been adopted in North America, South America, Taiwan, and South Korea. This system adapts trellis coding and 8-level vestigial sideband (8-VSB) modulation. The DVB-T system has largely been adopted in Europe, the Middle East, Australia, as well as parts of Africa and parts of Asia. The DVB-T system adapts coded orthogonal frequency division multiplexing (COFDM). The OFDM spread spectrum technique may be utilized to distribute information over many carriers that are spaced apart at specified frequencies. The OFDM technique may also be referred to as multi-carrier or discrete multi-tone modulation. This technique may result in spectral efficiency and lower multi-path distortion, for example. The ISDB-T system has been adopted in Japan and adapts bandwidth segmented transmission orthogonal frequency division multiplexing (BST-OFDM). The various DTTB systems may differ in important aspects; some systems employ a 6 MHz channel separation, while others may employ 7 MHz or 8 MHz channel separations.
While 3G systems are evolving to provide integrated voice, multimedia, and data services to mobile user equipment, there may be compelling reasons for adapting DTTB systems for this purpose. One of the more notable reasons may be the high data rates that may be supported in DTTB systems. For example, DVB-T may support data rates of 15 Mbits/s in an 8 MHz channel in a wide area single frequency network (SFN). There are also significant challenges in deploying broadcast services to mobile user equipment. Because of form factor constraints, many handheld portable devices, for example, may require that Printed Circuit Board (PCB)area be minimized and that services consume minimum power to extend battery life to a level that may be acceptable to users. Another consideration is the Doppler Effect in moving user equipment, which may cause inter-symbol interference in received signals. Among the three major DTTB systems, ISDB-T was originally designed to support broadcast services to mobile user equipment. While DVB-T may not have been originally designed to support mobility broadcast services, a number of adaptations have been made to provide support for mobile broadcast capability. The adaptation of DVB-T to mobile broadcasting is commonly known as DVB handheld (DVB-H). The broadcasting frequencies for Europe are in UHF (bands IVIV) and in the US, the 1670-1675 MHz band that has been allocated for DVB-H operation. Additional spectrum is expected to be allocated in the L-band world-wide.
To meet requirements for mobile broadcasting the DVB-H specification supports time slicing to reduce power consumption at the user equipment, addition of a 4 k mode to enable network operators to make tradeoffs between the advantages of the 2 k mode and those of the 8 k mode, and an additional level of forward error correction on multi-protocol encapsulated data—forward error correction (MPE-FEC) to make DVB-H transmissions more robust to the challenges presented by mobile reception of signals and to potential limitations in antenna designs for handheld user equipment. DVB-H may also use the DVB-T modulation schemes, like Quadrature Phase Shift Keving (QPSK) and 16-quadrature amplitude modulation (16-QAM).
While several adaptations have been made to provide support for mobile broadcast capabilities in DVB-T, concerns regarding device size, cost, and/or power requirements still remain significant constraints for the implementation of handheld portable devices enabled for digital video broadcasting operations. For example, typical DVB-T tuners or receivers in mobile terminals may employ super-heterodyne architectures with one or two intermediate frequency (IF) stages and direct sampling of the passband signal for digital quadrature down-conversion. Moreover, external tracking and Sound Acoustic Wave (SAW) filters may generally be utilized for channel selection and image rejection. Such approaches may result in increased power consumption and high external component count, which may limit their application in handheld portable devices. As a result, the success of mobile broadcast capability of DVB-T may depend in part on the ability to develop TV tuners that have smaller form factor, are produced at lower cost, and consume less power during operation. Furthermore, process and temperature variations within conventional tuners or receivers in mobile terminals result in deviation in the characteristics of many sub-circuits of the transceiver. A very important case is the deviation of the frequency response of analog filters used within the tuners or receivers. Such deviation of the frequency response results in deterioration of channel selection capabilities of the tuners or receivers.
As mobile terminals support a wider range of content from voice to data to video, they may be required to receive a correspondingly wider range of frequencies. Consequently, filtering circuitry may be required to filter signals for correspondingly wider ranges of frequencies.
FIG. 1 is diagram for a conventional filter calibration scheme utilizing a matched oscillator. This is an indirect filter calibration techniques, meaning that filter bandwidth is calibrated through the calibration of a circuit other than the filter itself (the oscillator). Referring to FIG. 1, there is shown a filter 202, an oscillator 204, a crystal oscillator 206, a frequency divider block 208, an exclusive-or (XOR) block 210, and a control block 212.
The oscillator 204 may comprise suitable logic, circuitry, and/or code that may enable generation of a clock signal. The oscillator 204 may comprise resistive (R) and capacitive (C) components. The R and C components may be variable or fixed. When the frequency associated with the clock signal is based on the values for the R and C components, the oscillator 204 may comprise an RC oscillator circuit.
The oscillator 204 may comprise active components, for example operational amplifier (op-amp) and C components. The op-amp component may comprise one or more electrical devices characterized by one or more transconductance (Gm ) values. The C component may comprise one or more electrical devices characterized by one or more fixed or variable capacitive values. When the frequency associated with the clock signal is based on the values for the op-amp and C components, the oscillator 204 may comprise a GmC oscillator circuit.
The frequency divider block 208 may comprise suitable logic, circuitry, and/or code that may enable generation of an output signal based on an input signal, wherein the input signal is characterized by a frequency that is a multiple of the corresponding frequency of the output signal. The value of each corresponding frequency may be determined by the frequency divider block 208.
The XOR block 210 may comprise suitable logic, circuitry, and/or code that may enable generation of an output signal in which the value of the output signal is based on a comparison of respective values associated with two input signals. The XOR block 210 may output a LOW value when the respective values of the two input signals are approximately equal. The XOR block 210 may output a HIGH value when the respective values of the two input signals are not approximately equal.
The control block 212 may comprise suitable logic, circuitry, and/or code that may enable generating a control signal, fControl, based on an input signal. The control signal may comprise an analog signal, such as a value for a voltage or a current for example, based on the input signal. The control signal may comprise a digital representation comprising one or more bits for example, based on the input signal. The control block 212 may receive an input signal from an external circuit. The control block 212 may generate the control signal based on the input signal. The control signal may be communicated to control at least a portion of the circuitry from which the input signal was received.
In operation, the crystal oscillator 206 may enable generation of a crystal (xtal) timing signal. The crystal timing signal may be characterized by a crystal frequency, fxtal. The frequency divider 208 may receive the crystal timing signal as an input signal. The frequency divider 208 may utilize a frequency division factor, fD, to generate a reference timing signal characterized by a reference frequency, fRef, and a reference phase φRef. The value of the reference frequency may be about equal to the ratio of the value of the reference frequency and the value of the frequency division factor, fRef/fD.
The oscillator 204 may enable generation of an oscillator timing signal characterized by an oscillator frequency, fOsc, and an oscillator phase φOsc. For an oscillator 204 comprising an RC oscillator, the oscillator frequency may be referred to as an RC oscillator frequency, fOsc(RC). The corresponding oscillator phase may be referred to as an RC oscillator phase, φOsc(RC). The value of the RC oscillator frequency and/or phase may be based on values for the R and C components. The values for the R and/or C components may be determined based on the control signal fControl.
The XOR block 210 may concurrently compare a value for the reference timing signal and a corresponding value for the oscillator timing signal at various time instants. Based on the comparison, the XOR block 210 may generate a difference signal. The difference signal may be nonzero when there are differences between the frequencies fRef and fOsc(RC), at a given time instant. The difference signal may be nonzero when there are differences between the phases φRef and φOsc(RC), at a given time instant.
The control block 212 may receive the difference signal and generate the control signal, fControl, based on the value of the difference signal. The control block 212 may communicate the control signal, comprising feedback information, to the oscillator 204. The feedback information may cause the oscillator 204 to adjust the R and/or C values. As a result of the adjustment, the corresponding frequency and/or phase values, fOsc(RC)/φOsc(RC) may be adjusted.
For an oscillator 204 comprising an GmC oscillator, the oscillator frequency may be referred to as an GmC oscillator frequency, fOsc(GmC). The corresponding oscillator phase may be referred to as an GmC oscillator phase, φOsc(GmC). The value of the GmC oscillator frequency and/or phase may be based on values for the op-amp and C components. The values for the op-amp and/or C components may be determined based on the control signal fControl.
The difference signal generated by the XOR block 210 may be nonzero when there are differences between the frequencies fRef and fOsc(GmC), at a given time instant. The difference signal may be nonzero when there are differences between the phases φRef and φOsc(GmC), at a given time instant.
The control block 212 may receive the difference signal and generate the control signal, fControl, based on the value of the difference signal. The control block 212 may communicate the control signal, comprising feedback information, to the oscillator 204. The feedback information may cause the oscillator 204 to adjust the Gm and/or C values. As a result of the adjustment, the corresponding frequency and/or phase values, fOsc(GmC)/φOsc(GmC), may be adjusted.
The oscillator 204 may utilize shared or common components with the filter 202. For example, for an oscillator 204 that comprises R and C components, the filter 202 may comprise equivalent R and C components. When the value for the f−3 dB filter cut-off frequency is based on the values of the R and C components, the filter 202 may comprise an RC filter circuit. For an oscillator 204 that comprises op-amp components and C components, the filter 202 may comprise equivalent op-amp and C components. When the value for the f—3 dB filter cut-off frequency is based on the values of the op-amp and C components, the filter 202 may comprise a GmC filter circuit.
For a filter 202 comprising an RC filter circuit, the control signal, fControl, generated by the control block 212 may cause the filter 202 to adjust the R and/or C values for the equivalent R and/or C components. As a result of the adjustment, the corresponding value for the f−3 dB filter cut-off frequency may be adjusted. For a filter 202 comprising a GmC filter circuit, the control signal, fControl, generated by the control block 212 may cause the filter 202 to adjust the Gm and/or C values for the equivalent op-amp and/or C components. As a result of the adjustment, the corresponding value for the f−3 dB filter cut-off frequency may be adjusted.
The oscillator 204 may be utilized to calibrate the filter 202 since the control signal, fControl, is generated based on the oscillator frequency fOsc, and/or oscillator phase φOsc. The control signal may cause the filter 202 to compute a value for the f−3 dB filter cut-off frequency. A disadvantage in this method is that the accuracy of the calibration may be limited based on the extent to which the values for the R and C components in the oscillator 204 are equal to corresponding values for the equivalent R and C components in the filter 202, for a given value of the control signal fControl. The accuracy of the calibration may also be limited based on the range of values for frequency, fRef, and/or phase, φRef, which may be generated by the frequency divider block 208.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.